Improve the travis configuration
Improve the travis configuration

- switch to the container-based infrastructure
- use the composer cache between builds
glutamatt/SwarrotBundle,odolbeau/SwarrotBundle,atailouloute/SwarrotBundle,jderusse/SwarrotBundle,swarrot/SwarrotBundle,atailouloute/SwarrotBundle,odolbeau/SwarrotBundle,glutamatt/SwarrotBundle,olaurendeau/SwarrotBundle,olaurendeau/SwarrotBundle,jderusse/SwarrotBundle,jderusse/SwarrotBundle,olaurendeau/SwarrotBundle,atailouloute/SwarrotBundle,odolbeau/SwarrotBundle,metfan/SwarrotBundle